<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <link rel="stylesheet" href="../source/reset.css">
</head>
<style>
  .claerfix::after{
    content: "";
    clear: both;
    display: block;
  }
  .header{
    color: #999;
    background: #e3e4e5;
    height: 40px;
    line-height: 40px;
  }
  .header .topnav>li{
    float: left;
    margin: 0 20px;
    width: 150px;
    height: 40px;
    text-align: center;
    box-sizing: border-box;
    position: relative;
  }

  .header .topnav>li:hover{
    background-color: #fff;
    border: 2px solid #ccc;
    border-bottom: none;
    line-height: 36px;
  }
  .header .topnav>li .submenu{
    text-align: left;
    line-height: 1.5;
    width: 300px;
    display: none;
    border: 2px solid #ccc;
    box-sizing: border-box;
    position: absolute;
    right: -2px;
    background-color: #fff;
  }

  .header .topnav>li:hover .submenu{
    display: block;
  }
  .header .topnav>li:hover::after{
    content: "";
    position:absolute;
    
    width: 100%;
    height: 5px;
    bottom: -2px;
    left: 0;
    background-color: #fff;
  }
</style>
<body>
  <header class="header">
    <ul class="topnav claerfix">
      <li><a href="">Ratione?</a></li>
      <li>
        <a href="">客户服务</a>
        <div class="submenu">
          <ul>
            <li>Lorem.</li>
            <li>Odit.</li>
            <li>Similique!</li>
            <li>Provident?</li>
            <li>Accusantium.</li>
          </ul>
        </div>
      </li>
    </ul>
  </header>
</body>
</html>